(1) Field of the Invention
The invention relates to compressors. More particularly, the invention relates to sealing of economized screw-type compressors.
(2) Description of the Related Art
Screw type compressors are commonly used in air conditioning and refrigeration applications. In such a compressor, intermeshed male and female lobed rotors or screws are rotated about their axes to pump the working fluid (refrigerant) from a low pressure inlet end to a high pressure outlet end. During rotation, sequential lobes of the male rotor serve as pistons driving refrigerant downstream and compressing it within the space between an adjacent pair of female rotor lobes and the housing. Likewise sequential lobes of the female rotor produce compression of refrigerant within a space between an adjacent pair of male rotor lobes and the housing. The interlobe spaces of the male and female rotors in which compression occurs form compression pockets (alternatively described as male and female portions of a common compression pocket joined at a mesh zone). In one implementation, the male rotor is coaxial with an electric driving motor and is supported by bearings on inlet and outlet sides of its lobed working portion. There may be multiple female rotors engaged to a given male rotor or vice versa.
When one of the interlobe spaces is exposed to an inlet port, the refrigerant enters the space essentially at suction pressure. As the rotors continues to rotate, at some point during the rotation the space is no longer in communication with the inlet port and the flow of refrigerant to the space is cut off. After the inlet port is closed, the refrigerant is compressed as the rotors continue to rotate. At some point during the rotation, each space intersects the associated outlet port and the closed compression process terminates. The inlet port and the outlet port may each be radial, axial, or a hybrid combination of an axial port and a radial port.
As the refrigerant is compressed along a compression path between the inlet and outlet ports, sealing between the rotors and housing is desirable for efficient operation. To increase the mass flow in a screw compressor an economizer is used. Typical economizer ports are located along the rotor length, positioned to become exposed to the compression pockets just after such pockets are shut off from the associated suction ports. At this location the refrigerant gas trapped within the rotors is near suction pressure. Connecting gas at a pressure above suction to the economizer ports allows for a quantity of gas to flow into the compressor. Furthermore, the feeding of gas into the rotors after suction is cut off increases the pressure of the trapped gas in the rotors. This reduces the amount of work required by the compressor. Also the economizer flow is above suction pressure, so the power for a given total refrigerant mass flow is reduced.
The suction port for a screw compressor can be axial, radial or a combination of both. The radial suction port cutoff is defined by the bore surrounding the rotor. The axial port is closed by the meshing of the screw rotors. Typical designs with both axial and radial suction ports require that the axial port be closed before or at the same time the radial port is closed.
To make the compressor more compact, shorter screw rotors are desirable. Also, using multiple female rotors about a single male rotor or multiple male rotors about a single female rotor may result in a shorter rotor set. By shortening the length of the rotors, the compression path gets shorter, which minimizes the opportunity and time required/available to inject economizer flow into the rotors.
Nevertheless, there remains room for improvement in the art.